# settings.py
...
AUTHENTICATION_BACKENDS = ('project.auth_backend.CustomBackend',)
...
# auth_backend.py
from django.contrib.auth.backends import ModelBackend
from django.contrib.auth.models import User

class CustomBackend(ModelBackend):
    def authenticate(self, username=None, password=None):
        try:
            user = User.objects.get(username=username)
            if user.check_password(password):
                [b]# do some voodoo[/b]
                return user
        except User.DoesNotExist:
            return None